Actually the only area in the game that seems relatively dark is the first couple rooms in the intro. The rest of the game came off as PG13 (haven't fully done swamp or touched underdark).
I was actually thinking the game would be more of how that was before I picked it up.
You didnt visited Goblin camp yet, did you?
I did, just a munch of destroyed junk.
edit* now that I think of it, there was the whole gnoll cut scene and area.
There are a lot of dead bodies, goblins eating dead adventurers, goblin children kicking a dead body (and I THINK you can kill those kids), the whole kicking people into a spider pit, animal cruelty with the owlbear cub, slavery, torture, and in general things that would keep the game's rating Mature and other area's equivalents of Mature.
With the Gnoll, if you got the symbol of the absolute you can make her eat the Zhents, eat her own pack, and then make her eat herself (though due to my like of gnolls I'd rather somehow recruit her with the tadpole).
You can doublecross the Zhentarim which actually leads to the guy you are doublecrossing them with being captured, tortured, and then executed.
You can pick the evil side and slaughter the tieflings. You can also murder people and then try to speak with their corpses.
And worst of all, you can be a dick to the dog.
Overall there are fairly dark and bloody parts of the game. Further on in the swamp there is the Hag which is a very typical hag with her basement of people she has "helped".